X-ray imaging methods can be divided into different subtypes on the basis of many different criteria. For example, full-field imaging generally refers to an imaging method wherein an object is irradiated by a single stationary exposure using an X-ray beam of a size equal to the size of the object and the image information is correspondingly detected by means of an image information receiver of a size equal to or in practice somewhat larger than the object to be imaged. In this context, digital imaging therefore has to be implemented using a detector of the size corresponding to what would be used in case of traditional film imaging. However, detectors having a large image forming surface are expensive, and consequently, depending on the imaging application, it is sometimes more justifiable to use a narrow detector and e.g. an imaging method wherein the object to be imaged is scanned with a narrow radiation beam while the detector is moved on the other side of the object in synchronism with the scanning movement of the radiation beam.
A generally known practice in X-ray imaging is to use radiation-sensitive semiconductor detector surfaces whose basic structure consists of tiny image elements, i.e. pixels, wherein the radiation absorbed in the area of the pixels is first converted to a wavelength of visible light and further into electric signals. Today there are also detectors based on direct detection of X-radiation, wherein X-ray quanta, on being absorbed into the quanta-absorbing medium of the detector, are converted directly into electron-hole pairs, i.e. into charges detectable by electric means. Such media include e.g. biased (photoelectric) Ge, Si, Se, GaAs, HgI, CdTe, CdZnTe or PbI semiconductor materials. Detector elements of this type can be divided into pixels e.g. by using an electric field arranged over them in an appropriate manner such that each one of the electron-hole pairs produced can be collected, avoiding lateral migration, in the area of its respective pixel. Using this kind of technology, it is possible to achieve very high quantum efficiency (dqe), yet without compromising on resolution.
In connection with digital imaging it is not at all rare that a detector technology which is suited for use in a given imaging application or in given imaging applications is poorly or not at all applicable for some other type of imaging. For example, a detector technology designed for taking full-field/radiography images is not necessarily applicable for use in tomography imaging, where the aim is to obtain an image of a layer of an object, and vice versa. There are also differences between tomography techniques due to which a detector technology applicable for use in one technique is not necessarily applicable for use in another technique. A fundamental difference between imaging methods relates to the read-out of information from the detector: in some cases it is required that read-out of image information from the detector should be possible during imaging, whereas in other cases the information is only read out after exposure.
In so-called Frame Transfer (FT) technology, the image information of the entire pixel matrix is repeatedly transferred during exposure to a “shelter” for actual information read-out, i.e. away from the area where image information is received. An FT detector may be based on e.g. so-called CCD or CMOS detector technology, and it can be constructed in several different ways. A typical solution is to divide a CCD element into two sections in such a way that a first section is used for detection of image information while the other section is fitted in a place protected from radiation. In this case, information is transferred in a periodic manner during exposure from the first detector section to the other for read-out, while integration of information still goes on in the detector section unprotected from radiation. However, this gives rise to the problem typically associated with FT imaging that the information integrated in the pixels during transfer of image information causes undesirable blurring of the (partial) image being transferred to storage. Moreover, many detector solutions designed for FT imaging are ill-suited or completely unsuited for use in more than one type of imaging.
In principle, it might be possible to avoid the above-discussed undesirable blurring of the image by always momentarily interrupting either the irradiation of the object or the integration of the information for the duration of transfer of image information. However, in many applications, always interrupting irradiation for the duration of information transfer is an unrealistic alternative difficult to implement. On the other hand, interruption of integration again would lead to a loss of that portion of the information descriptive of the object being imaged which is produced by irradiation of the object during integration. This would naturally be a quite unsatisfactory solution e.g. in conjunction with x-ray imaging of humans, wherein all unnecessary irradiation of the object to be imaged is undesirable or even forbidden by orders of the authorities. On the other hand, depending on the detector technology being used, interruption of integration for the duration of information readout would not necessarily be even technically possible. Further, if the imaging method additionally involves some specific mutual relative movement between the object and the imaging means, then the situation is even more problematic as in that case it would in principle be necessary to stop such movement as well for the duration of interruption of irradiation or integration.
Patent specification U.S. Pat. No. 6,847,040 discloses a detector solution which, among other things, makes possible both full-field imaging and scanning imaging implemented by the TDI technique using a technology whereby the image information produced by X-ray quanta having penetrated the object to be imaged is detected by counting the number of X-ray quanta absorbed. In the detector described in this specification, pixel values are shifted by loading the counters from the counters of the preceding column, and the information is read out from the last column via a shift register arranged at the edge of the detector. The detector technology described in the specification is especially designed to enable imaging by the so-called TDI technique, in which it is required that pixel values can be shifted and integration of image information carried on pixel column by pixel column as a function of the speed of propagation of the radiation beam in the object. However, the technique in question is not optimal e.g. for a type of imaging in which the information integrated in the counters at a given instant of time should be recovered in real time during exposure, because during the column-by-column shifting of the information there would also be integrated in the pixel values other information besides that descriptive of those points of the object intended to become imaged.
In detectors employing counters, the read-out of image information can be implemented using read-out electronics arranged on the surface of an amorphous silicon substrate. The detector can also be arranged to consist of smaller modules and the read-out electronics can be implemented using CMOS (Complementary Metal-Oxide Semiconductor) technology. For example, in a detector of the type presented in WO specification 98/16853, it is possible to arrange for each pixel to be selected at a time and for the information to be read out via a signal bus extending to the edge of the detector. The arrangement makes it possible, among other things, to read out the information from individual pixels in real time during exposure. However, considering a pixel matrix consisting of a plurality of pixels as a whole, the larger the matrix, the larger is always the time difference between the information of the pixel of the matrix read out first and that of the pixel read out last. Such a non-simultaneity may cause distortions in the image being formed that in practice prevent this detector technology in question from being used for real-time imaging, i.e. for imaging where information is read out from the detector without interrupting the exposure.
When prior-art technologies as described above are used, and especially in the case of a larger pixel matrix, reading out the information from the detector inevitably always takes that much time that distortions may be produced in the image being formed. The use of detector technology employing counters does not in itself eliminate the above-described problems relating to real-time imaging.